RBCF-1 cells derived from the caudal fin of the goldfish (Carassius auratus) have been established at 37 degrees C, and could grow at a wide range of temperatures (20-37 degrees C). To examine the mechanism(s) of this eurythermic property, we isolated clones from RBCF-1 cells with different optimal growth temperatures. R37 clone grew continuously at 37 degrees C but could not grow at 27 degrees C. R27 clone grew continuously at 27 degrees C but could not grow at 37 degrees C. Thus R37 and R27 cells may be referred to as stenothermic. The R3727 clone, which was derived from R37 cells selected at 27 degrees C, and the cell hybrids of R37 and R27 cells, could proliferate for a long time and form colonies at 27-37 degrees C, like progenitor RBCF-1 cells, so they may be referred to as eurythermic. The shape of colonies of R3727 cells changed with incubation temperature; they resembled those of R37 cells at a higher temperature (37 degrees C), while at a lower temperature, (27 degrees C), they were like those of R27 cells. The lethal effects of heat treatment were compared among RBCF-1 clones. R27 cells were most sensitive, and R37 cells were most resistant. The heat treatment could induce the four major heat-shock proteins (hsp30, hsp42, hsp70 and hsp90). Marked induction of hsp70 and hsp30 was observed at 33 degrees C for R27, at 37 degrees C for R3727 and at 40 degrees C for R37. With R3727 cells, induction of hsps and thermosensitivity were highly dependent on the incubation temperature before heat treatment.